The present invention relates to a multistage spur-type gearing for small and miniature electric motors, comprising a gearing housing including an input shaft opening through which a motor shaft can be inserted into the interior of the housing for operating the spur-type gearing with the small or miniature electric motor, at least two gear shafts that are held in the gearing housing and extend in parallel with a center axis of the gearing, and spur gears of different diameters which are arranged on the gear shafts, at least one of the gear shafts being fixedly held as a gear axle in the gearing housing, and at least two spur gears being fixedly connected to one another and combined to form at least one dual gear set which is rotatably supported on a gear shaft or axle.
Small and miniature electric motors are electric motors, most of the time d.c. motors, with small to very small dimensions, normally a diameter below 50 mm and a length below 150 mm, and a high motor output per weight. Small and miniature electric motors often rotate at a very fast speed and are normally used at speeds around 10,000 r.p.m. and more. To reduce the high speeds of said motors, use is made of spur-type gearings of a corresponding size which most of the time are directly flanged onto the motor housing. The transmission or gear ratios of the gearings or transmissions, which are respectively adapted to a specific motor, are often preset by the works for the customer. Motor and gearing can be ordered together and are also supplied jointly, being already pre-assembled as a unit. That is why the conventional spur-type gearings are most of the time provided with a cylindrical housing whose outer diameter corresponds to that of the associated motor. The front sides of the gearing are formed by flanges with which the gearing can be fastened to the motor or to another device.
In conventional spur-type gearings, the gear shafts are rotatably supported in the front flanges of the gearing housing. Each gear shaft has fixedly mounted thereon a spur gear which is in meshing engagement with a spur gear of a different diameter that is arranged on another gear shaft. In the conventional spur-type gearings, each of the spur gears is thus integrally rotating with the associated shaft. In conventional multistage spur-type gearings two spur gears of different diameters, a toothed gear and a pinion gear are thus arranged on a gear shaft at the most. The distance of the gear shafts of one reduction stage is defined by the gear ratio of the respective reduction stage.
In practice, the typical high speeds of small and miniature electric motors require a high reduction, i.e. a high ratio of input speed to output speed of the gearing. Such reduction ratios cannot be realized in a single gear stage on account of the predetermined small constructional dimensions. Therefore, in conventional spur-type gearings, the total gear ratio is distributed over several gear or transmission stages, so that at least two gear shafts are provided for.
A considerable drawback of the conventional structure of spur-type gearings is that the available space cannot be used in an optimum manner per reduction stage. Every additional reduction stage requires the installation of an additional gear shaft, which is bound to enlarge the gearing.
A further drawback of the gearings or transmissions normally used for small and miniature electric motors is the relatively large space required by said gearing at high reduction ratios, which is due to the fact that each gear shaft only carries one pair of toothed gears fixedly connected to said gear shaft. Therefore, a separate shaft has to be provided for each pair of toothed gears, which in the end leads to a relatively large number of parts in said conventional spur-type gearings. This creates relatively high costs during production and assembly.
To avoid said problems, the prior art discloses spur-type gearings in which at least two spur gears are rigidly connected to one another and combined to form at least one dual gear set. The dual gear set is rotatably supported on a gear shaft or axle. Since the dual gear set is supported in a rotatable manner, the gear axle or shaft can accommodate more than one transmission stage. As a result, a separate gear shaft need no longer be provided for every reduction stage.
For instance, DE 43 42 185 A1 describes a gearing having a number of meshing dual gear sets. A respective dual gear set is seated on a gear axle integrally formed by the gearing housing. However, the gearing according to DE 43 42 185 A1 has the drawback that the offset between motor and output shaft of the gearing is relatively large.
This problem is solved in the transmission of EP 371 690 A1 which is of a smaller constructional size than the transmission of DE 43 42 185 A1. However, this transmission is still relatively large, occupying more space than the motor itself. Because of their constructional size, the transmissions of EP 371 690 A2 and DE 43 42 185 A1 are not suited for today""s small and miniature electric motors which should combine a minimum volume with maximum performance.
This drawback has been eliminated in the transmission of FR 1,448,923. In a cut transverse to the motor shaft, the transmission described therein has about the same cross-sectional area as the electric motor. Since the motor shaft in the transmission of FR 1,448,923 itself forms a transmission shaft with spur gears, it is very troublesome to separate the motor from the transmission shown therein. Use of such a transmission on an industrial scale (where many different transmissions with different reduction stages are mounted on the motor in accordance with the customers"" desires just before delivery) is thereby made unprofitable. Furthermore, the transmission of U.S. Pat. No. 1,521,882 can also not be detached from the motor.
By contrast, U.S. Pat No. 2,831,364 shows a mechanical transmission which in the illustrated embodiment would not be useful because of the usual sizes of small and miniature electric motors and the high speeds thereof.
Finally, U.S. Pat. No. 2,908,180 forms the generic prior art for the subject matter of claim 1. This document shows a spur-type gearing which is screwed onto an induction motor. This gear mechanism has the drawback of a relatively large constructional volume in the radial direction. In this instance, too, the gear mechanism is larger than the associated electric motor.
It has been found in the case of all of the above-mentioned transmissions that there is a high wear at the high speeds as are arising during operation of today""s small and miniature electric motors together with the power density achieved today.
It is there fore the object of the present invention to improve the above spur-type gearings by simple constructional measures in such a manner that even at high speeds they are hardly subject to any wear and can have the same constructional size as the corresponding electric motors.
According to the invention this object is achieved for a multistage spur-type gearing of the above-mentioned type in that the gear shaft or axle is made from a hard material and that the input shaft opening is arranged to be essentially coaxial to the center axis of the gearing housing.
Gear shafts which are stationary relative to the gearing housing and do not transmit any torques are designated as gear axles. Dual gear set primarily means the connection of two spur gears at the front sides thereof and at a distance that is as small as possible.
This solution is simple and permits the construction of small gearings or transmissions even at extremely high reductions and at high input speeds.
According to the invention the problem of premature wear is solved in that at least one gear axle is made from a hard material; hard materials within the meaning of the technical teaching of this invention permit the substantially dry support of the typically rapidly rotating spur gears on the gear shaft or axle without any significant wear. According to tests performed by the inventor, such a condition is primarily met by materials having such a high hardness as is e.g. inherent to ceramic or hard-metal materials. However, the term hard materials should also comprise other materials or classes of materials whose characteristics, in particular the hardness thereof, are comparable to those of e.g. ceramic materials.
Suitable hard materials can be found easily by performing service life tests (known per se) under load and operational conditions. The test conditions in these tests expediently comply with those prevailing during operation of the gearing together with small and miniature electric motors. Suitable hard materials for the gear axles and shafts can be determined by way of such tests in that under such test conditions they yield similar service lives as ceramic or hard-metal gear axles or shafts.
The reason why the spur-type gearing according to the invention is constructionally so small in comparison with the prior art is that according to the invention the input shaft opening is arranged to be coaxial to the center axis of the gearing.
As a consequence, the drive power is centrally introduced into the gearing and the gear shafts and axles with the reduction stages can be arranged in a space-saving manner around the input shaft opening. On the one hand, the available space can thus be exploited in an optimum manner; on the other hand, such a design makes it possible to fasten the gearing housing directly to the electric motor without the gearing housing projecting therefrom.
In a particularly advantageous development of the invention, the hard material can substantially contain a ceramic material. In tests performed by the inventor, gear axles or shafts, in particular, made from ceramics achieved the longest service lives in the support of the dual gear sets.
A fast and inexpensive manufacture of the gear axles or shafts and thus an accompanying cost reduction of the whole transmission can be achieved when according to a further advantageous development the gear axles or shafts are produced in a sintering process from a sintered material. All of the common hard materials according to the above definition can be sintered.
Among the hard materials according to the invention, zirconium oxide has turned out to be particularly suited due to its hardness and, in particular, high resistance to wear. Gear axles or shafts of zirconium have a particularly long service life.
Rotating gear axles can substantially be dispensed with because of the dual gear sets which are rotatably supported on the gear axle. That is why at least one gear axle can be rigidly connected to the gearing housing. In comparison with rotatably supported gear shafts requiring a separate support, this will reduce the costs because larger tolerances are acceptable for the accommodation of the gear axles in the housing in the case of a fixed connection, for instance by gluing, pressing or injection. The manufacturing efforts are thereby reduced.
In a further advantageous development, the gear axles can be aligned in parallel with one another. Since the gear axles are normally held in flanges of cylindrical housings at the front side, a symmetrical structure of the flange is possible in the case of gear axles oriented in parallel with one another.
A preferred design of the invention can be obtained in that at least two dual gear sets are made identical. Such a construction yields a simple design of the gearing because, in contrast to the conventional spur-type gearings, the gearing of the invention may be composed of standardized toothed gear sets. Furthermore, in this design of the invention, a stepping down or a stepping up of the gearing can easily be changed by adding further dual gear sets.
Diametrically opposed dual gears exploit the available space not only in an optimum way, but also permit an easy fastening to a motor by means of two continuous screws arranged at a right angle relative to the gear axles.
In a further advantageous development the dual gear sets may comprise an integrally formed toothed gear section and an integrally formed pinion section, the pinion section having a smaller diameter than the toothed gear section. Such integral dual gear sets can be produced by means of known production methods, such as sintering or injection molding, from a great number of materials in a simple and inexpensive manner.
In a further advantageous development a dual gear set which consists of a hardened material, preferably a metal material, in particular sintered iron having a large open-pore volume, similar to a sintered bearing, has turned out to be useful in combination with the gear axle or shaft made from a hard material according to the invention. The combination of a hardened material for a spur gear and the ceramic shaft effects a particularly long service life. In contrast to the hub-shaft combinations that are standard in the construction of transmissions, the transmission axle is made from a harder material according to the invention than the toothed gear that is rotatably supported on the gear axle or shaft.
To permit a connection which can easily and rapidly be established between the electric motor and the spur-type gearing during operation with a small or miniature electric motor, the gearing housing may further be arranged to be coaxial to a motor shaft of the small or miniature electric motor during operation.
The power output from the small or miniature electric motor is first supplied to an input stage, i.e. a first meshing pair of spur gears. In a further advantageous development of the invention, the input stage which is in engagement with the motor shaft can be designed as a ring gear seated on a gear axle or shaft. With a small constructional size, a maximum reduction can already be achieved in the first stage with a ring gear so that the driven side of the input stage is already rotating at a slower speed and can thus be supported more easily on the gear axle or shaft. Moreover, optimum tooth engagement ratios are obtained that are equal to a helical toothing. Furthermore, the noise level can be considerably reduced by the xe2x80x9csofterxe2x80x9d tooth engagement. Since the load on the teeth is still small in the first stage, the ring gear can also be made from a suitable plastic material.
In a further advantageous embodiment, there may be provided an output shaft of the multi-stage spur-type gearing which is led to the outside for coupling the power transmitted by the gearing through an output shaft opening mounted on the gearing housing, and which is rotatable relative to the gearing housing. In a further advantageous development, the output shaft is formed at least sectionwise as a hollow shaft in which a gear axle or shaft is received at least in sections. This also effects a very small constructional size.
Furthermore, it may be advantageous that a gear axle or shaft is designed as an output shaft that is rotatably supported in the gearing housing. In this design a gear shaft serves both the support of dual gear sets rotatably held on said gear shaft and as an output shaft; a toothed gear at the output stage side must then of course be connected to said gear shaft in a substantially rigid manner.
In a further advantageous development, a bearing bush in which the output shaft is rotatably supported in the gearing housing may be arranged between the gear shaft and the gearing housing. A longer service life is thereby achieved with the spur-type gearing of the invention. In particular, the bearing bush may be made from a material adapted to ceramic axles or shafts, resulting in small coefficients of friction and little wear between the shaft and the ceramic axles. By contrast, a direct support of the ceramic shafts in the flanges would effect increased wear of the flange in the case of specific flange materials.